The invention relates to a fuel cell system comprising a fuel cell which includes an anode compartment and a cathode compartment which are separated from one another by a proton-conducting membrane.
At present, the method most widely envisaged for converting liquid energy sources into electrical energy in a fuel cell system comprising a proton exchange membrane (PEM fuel cell) all over the world is that of reforming methanol in a gas generation system. This involves a water/methanol mixture being evaporated and being converted, in a reformer, into hydrogen, carbon dioxide and carbon monoxide. Evaporation and reforming are very expensive in terms of the energy balance. This entails reduced efficiencies for the system as a whole. Moreover, gas beneficiation steps are required to clean the reforming gas. The cleaned gas is delivered to the PEM fuel cell system. Additionally, a cooler must be provided to cool the coolant/fuel mixture circulating in the anode circuit.
A further problem is that of the water used in the reforming process. The product water produced on the cathode side does not suffice to cover the water needed. Consequently, a separate water tank is required.
A so-called direct-methanol fuel cell system, as disclosed by U.S. Pat. No. 5,599,638, makes use of an aqueous methanol solution which reacts on the anode side to form carbon dioxide. The fuel cell system described there includes a so-called stack consisting of a plurality of interconnected fuel cells. The anode compartment of the stack forms part of an anode circuit, comprising a heat exchanger to cool the coolant/fuel mixture which is ducted off from the anode outlet and contains carbon dioxide, a circulation tank in which the cooled mixture is added to a freshly supplied coolant/fuel mixture, a gas separator which is integrated within the circulation tank and has the purpose of separating carbon dioxide, and a pump to feed the coolant/fuel mixture from the circulation tank into the anode compartment via a corresponding feeder. The oxygen- and water vapour-comprising cathode off-gas of the known fuel cell system is passed through a water separator, the separated water being fed to the coolant/fuel mixture which is to be delivered to the anode circuit, and part of the remaining oxygen being passed to the oxidant supply for the cathode compartment.
Based on this, it is an object of the invention to provide a simpler-design, compact fuel cell system comprising a proton-conducting membrane and having an improved overall efficiency.
In a preferred embodiment, the fuel cell system involves passing water through the anode compartment into the cathode compartment, evaporation cooling is effected in the fuel cell as the water is absorbed by the hot air of the cathode compartment, said evaporation cooling being utilized according to the invention to cool the anode circuit. Owing to this measure, the cooler which otherwise has to to be provided in the anode circuit can be dispensed with.
In a preferred method, the fuel cell is operated in heat balance equilibrium, i.e. the fuel cell is operated in a steady state at a temperature which, on the one hand, depends on the properties of the proton-conducting membrane and, on the other hand, can be adjusted via the speed of the liquid pump. Depending on the duty point, the temperature of the steady state operation is between 90 and 110xc2x0 C. Setting a steady-state operating temperature is of crucial importance in increasing the efficiency of the fuel cell or of the stack formed from a plurality of fuel cells, since this will enable isothermal operation of the stack, i.e. temperature differences over the length of the stack of an order of magnitude of about 10xc2x0 C., which are standard in known systems, will no longer occur, or only to an insignificant extent.
The inventive evaporation cooling in the fuel cell has the additional advantage that the mass flow of the dry air is increased by a factor of 1.5 to 2, entailing an increase in expander capacity by the same factor. This also entails energy savings for air supply in full-load operation.
In a preferred embodiment, an air cooler downstream of the expander is provided which is thermally coupled to the vehicle radiator and which serves for condensing out water to achieve a positive water balance in the system.